1. Field
This invention pertains generally to captive screw assemblies, and more particularly, to the design of the captive screw assembly.
2. Related Art
Captive screws are used to releasably attach panels over an opening or secure a door closed. Captive screws have the advantage that once the screw is disengaged from a threaded hole underlying the panel or door jamb to which the captive screw is attached, there are no loose parts that can get lost or fall into crevices in the equipment protected by the door or panel where the impact of the falling screw may cause damage to sensitive equipment. The captive screw is generally attached to a panel such that the captive screw is in alignment with a hole in the panel. The hole in the panel is aligned with a threaded hole in the structure underlying the panel when the panel is in the closed position. The screw is engaged to the threaded hole to secure the panel in the closed or attached position. The screw is disengaged from the threaded hole to allow the panel to be removed or the door opened.
FIG. 1 illustrates a cross section of a captive screw assembly 10. The captive screw assembly 10 includes a captive screw 12, a sleeve or ferrule 14, and an optional spring 16. As previously mentioned, the captive screw fastener 10 releasably secures a closure member in the closed position. As an example, the captive screw 10 can be used to secure the access panels of electrical equipment over the access openings in, for example, a PSA (Power Switch Assembly) in a nuclear power generation facility. The screw 12 has a screw head 18 and a shaft 20 having a threaded section 22. An annular flange 24 is provided on the shaft 20 intermediate the screw head 18 and the threaded section 22 of the shaft 20. The annular flange 24 or first flange may be spaced apart from the screw head 18 as shown in FIG. 1 or may be secured adjacent the screw head. The ferrule 14 has a hollow bore 26, a panel engaging end 28, and a screw retaining end 30. A portion of the screw shaft 20, including the annular flange 24 is housed within the bore 26 of the ferrule 14 with the annular flange 24 being captured between the ends 28 and 30 of the ferrule 14 by the ferrule internal shoulder 46 in the panel engaging end and the screw retaining end flange 40, respectively. The screw 12 is axially movable relative to the ferrule 14 between an extended position in which the annular flange 24 approaches the panel engaging end 28, and a retracted position in which the annular flange 24 approaches the screw retaining end 30. In addition, the screw 12 can rotate freely relative to the ferrule 14 about the screw's own central longitudinal axis. In the extended position, the threaded portion 22 of the screw shaft 20 projects from the panel engaging end 28 of the ferrule 14 to its maximum amount. The amount of projection of the threaded portion 22 of the screw shaft 20 from the panel engaging end 28 of the ferrule varies as the screw is moved relative to the ferrule. In the retracted position, the amount of projection of the threaded portion 22 of the screw shaft from the panel engaging end 28 of the ferrule is at its minimum. In some embodiments, the threaded portion 22 of the screw shaft may be completely retracted into the bore 26 of the ferrule 14 when the screw 12 is moved to the fully retracted position.
The screw head 18 is typically located outside the ferrule 14 where it can be accessed by a user's fingers for tightening or loosening the screw 12. The screw 12 may be provided with knurling or serrations on the peripheral surface 32 of the screw head 18 to give the user's fingers a better grip. The panel engaging end 28 of the ferrule 14 is designed to engage a hole 34 in the panel 36; the panel 36 being the panel that is to be secured by the captive screw assembly 10, in order to attach the ferrule 14 to the panel 36. The screw retaining end 30 has a hole 38 that allows the shaft 20 of the screw 12 to extend through the hole 38 and into the bore 26, however, the diameter of the hole 38 in the screw retaining end of the ferrule is less than the diameter of the annular flange 24 such that the annular flange 24 remains captured within the bore 26 of the ferrule 14. The screw retaining end 30 of the ferrule may be formed by a second annular flange 40 which projects from the peripheral wall 42 of the ferrule 14 inward toward the central longitudinal axis of the ferrule. The panel engaging end 28 of the ferrule 14 also has a hole 44 that provides clearance for the shaft 20 of the screw but is too small in diameter to allow the annular flange 24 of the screw to clear the opening 44 in the panel engaging end 28 of the ferrule. Peripheral to the hole 44 in the panel engaging end of the ferrule is a shoulder 46 which acts as a seat for one end of the optional spring 16 that may be provided intermediate the flange 24 of the screw and the panel engaging end 28 of the ferrule. This optional spring 16 acts to bias the screw 12 toward the retracted position once the screw is disengaged from the threaded hole (not shown) in the structure underlying the panel 36. The captive screw assembly 10 is attached to a panel 36 such that the screw 12 is in alignment with the hole 34 in the panel 36. The hole 34 in the panel 36 aligns with a threaded hole (not shown) in the structure underlying the panel 36 when the panel 36 is in the closed position. The screw 12 is engaged to the threaded hole to secure the panel 36 in the closed or attached position. The screw 12 is disengaged from the threaded hole to allow the panel 36 to be removed.
In the prior art embodiment illustrated in FIG. 1, the panel engaging end 28 is formed by a portion of the ferrule which has reduced internal and external diameters. This reduced diameter portion is then flared outward giving the panel engaging end 28 a U-shaped cross section with the concave side of the “U” shape facing outward away from the central longitudinal axis of the ferrule. The reduced diameter portion of the panel engaging end 28 also forms an exterior shoulder 48 which abuts the surface of the panel 36 once the captive screw assembly 10 is mounted to the panel 36. The ferrule 14 can be pressed in or crimped to the hole 34 such that the surface 50, which surrounds the bottom rim 52 of the panel engaging end 28, is pressed against the sidewall of the hole 34 to thereby press fit the ferrule 14 to the panel 36. The panel engaging end 28 may be made in a variety of different configurations such as the press-in type depicted in FIG. 1, the snap-in type, the threaded type, the swaged type, etc.
It is sometimes difficult to assure that the captive screw assemblies are securely attached to the panel cover to avoid the captive screw assemblies from being dislodged and possibly dropping into sensitive instrumentation within the cabinets, potentially damaging the instrumentation.
According, means are desired that can adequately secure the captive screw assemblies to assure they will not be readily dislodged. Furthermore, such a means is desired that will produce a consistent result.
Additionally, a means is desired that will disengage such captive screw assemblies that have been securely attached if they become damaged or otherwise are unable to fulfill their intended function.
Further, a new captive screw assembly is desired that will enhance the attachment coupling between the screw assembly and the panel cover.